1. Field of the Invention
The present invention pertains to providing output feedback to a regulator, particularly to a regulator for a switch mode power supply.
2. Related Art and Other Considerations
Various electronic devices utilize a regulated, switch mode power supply circuit. In a switch mode power supply, D.C. power is supplied and therefore requires some type of switching to couple it across a transformer. Regulation is advantageous so that the power supply need not deliver its peak power level at all times to the load, which might result in losing the unused power (which is the difference between the worst case peak power and the current requirement) in the form of heat.
The desire for regulation requires that the voltage output of the power supply circuit be observable. Traditional forms of providing this output feedback are a separate transformer or an optical isolator. Either of these solutions necessitates additional cost, e.g., either an additional winding on the pancake transformer or a LED/photo transistor pair.
What is needed, therefore, and an object of the present invention, is a technique for generating feedback without the use of additional transducers or optical isolators.
A power supply circuit for supplying power to a load comprises a regulator, a switching transistor connected to the regulator; and a power transformer connected between the switching transistor and the load. The regulator is of a type that requires feedback regarding the amount of voltage supplied to the load. In accordance with the present invention, a feedback signal to the regulator is generated by monitoring an input to the power transformer. In particular, a voltage signal related to a voltage on the primary of the power transformer after a voltage spike is applied as the feedback signal to the regulator. A peak detector detects the voltage spike and applies the voltage signal to the regulator. A peak detector reset circuit resets the peak detector.
The peak detector preferably comprises a transistor connected as an emitter follower between the primary of the power transformer and ground. A collector of the power transformer is connected to the primary of the power transformer and an emitter of the transistor is connected to the regulator.
In one example context of usage, a variable air gap exists between a stationary primary of the power transformer and a secondary of the power transformer. For example, the secondary of the power transformer can be mounted with the load on a rotatable element, such as a rotating drum of a magnetic tape drive. A zener diode is connected in parallel across the secondary of the power transformer.
Thus, in the switch mode power supply of the invention, feedback for its operating level is taken from a driven transformer primary, simplifying the system in terms of both cost and complexity.
Moreover, a method of calibrating the supply advantageously does not require additional test equipment, is simple, provides a robust operating point. One example employment of the power supply of is for use on a rotating head magnetic storage device, in which the calibration method can be accomplished using firmware by the drive in the field. In accordance with the calibration method, a zener diode is connected in parallel a cross the secondary of the power transformer. An operating voltage point of the regulator is set to a point just before a voltage at which the zener diode turns on.